1. Field of the Invention
The present invention relates to a variable capacity wobble plate type refrigerant compressor capable of varying a capacity of a compression of a refrigerant gas by varying the reciprocation strokes of compressing pistons by changing an angle of inclination of a non-rotatably inclinable wobble plate supported by a drive plate rotating with a drive shaft when the drive shaft is driven by an external drive source, such as a vehicle engine, via a solenoid-operated electro-magnetic clutch. More particularly, the present invention relates to the above-defined type of refrigerant compressor provided with a highly accurate capacity detector generating electric signals indicating a precise current capacity at which the compressor is currently operated.
2. Description of the Related Art
A variable capacity wobble plate type compressor with a solenoid-operated wobble angle control valve is disclosed, for example, in U.S. Pat. No. 4,747,754 to Fujii et al. The solenoid-operated wobble angle control valve of the compressor of U.S. Pat. No. 4,747,754 is incorporated in a compressor head to promote a smooth and quick change of wobble angle of a wobble plate within a crankcase connected to the compressor head, in response to a signal indicating a change in a refrigerating load.
Japanese Unexamined (Kokai) Patent Publication No. 62-218670, published on Sept. 26, 1987, discloses another variable capacity wobble plate type refrigerant compressor provided with a capacity detector which comprises an electro-magnetic induction type detecting device fixedly attached to an outer surface of a frame member of the compressor, and a pin-shape detected element attached to a wobble plate. When the compressor is operated, the wobbling plate carries out a wobbling motion by which the detected element is reciprocably moved to pass the detecting device. The detecting device then generates an electric pulsive signal each time the device electro-magnetically sensed the passing of the element, and the electric pulsive signal is sent to a control unit to which the detecting device is electrically connected. The control unit thus detects a number of revolutions and an extent of the angle of wobbling of the wobble plate on the basis of the pulsive signals sent from the detecting device. More specifically, the control unit detects a time period T.sub.2 for which the detected element is moved in one region arranged on one of the right and left sides with respect to the center of the detecting device, and a time period T.sub.1 for which the detected element is moved in another region arranged on the other side, on the basis of the time interval between two adjacent electric pulsive signals. The control unit further calculates, from the detected T.sub.1 and T.sub.2, a ratio T.sub.1 /T.sub.2 by which the strokes of the compressing pistons are detected, and therefore, the number of revolutions and the extent of the angle of wobbling of the wobble plate are detected, and as a result, the capacity of the compressor at the current operating condition thereof is detected.
Although Japanese Unexamined (Kokai) Patent Publication No. 62-218670 does not disclose the material used for the pin-like detected element, it will be obvious to a person skilled in the art that, when the detected element is a permanent magnet, the construction of the detected element per se and the detecting device is simple. Nevertheless, when an experiment was conducted by the inventors with respect to one example of a wobble plate type compressor provided with a capacity detector using a detected element made of a permanent magnet, it was found that electric signals, i.e., electric voltage signals output by a detecting device, contain noise, and accordingly, a train of electric pulsive signals derived from the electric voltage signals were inaccurate due to the existence of erroneous pulse signals in the train. Since both erroneous and true electric pulsive signals were sent to a control unit, the control unit generated erroneous information with regard to the compressor capacity to be sent to an electronic microcomputer controlling the overall operation of the wobble plate type compressor in response to various external signals, such as a cooling load, atmospheric temperature, and so on. When a further experiment was conducted to determine the cause of the generation of the above-mentioned erroneous information, it was found that a magnetic flux leaking from a solenoid clutch device mounted on the compressor end periodically acts on the detecting device via a steel drive shaft of the compressor and a steel support arm member for supporting a drive and wobble plate assembly on the drive shaft, to thereby cause the output of the unwanted electric voltage noise from the detecting device. A further explanation of this problem will be given below with reference to FIGS. 3A through 3C.
Note, the detecting device used for the experiment conducted by the present inventors comprised an electro-magnetic induction type magnetic-flux sensor and an electric binary circuit element for converting an output signal of the magnetic sensor into electric binary signals.
FIG. 3A indicates electric voltage signals output by the detecting device. As shown in FIG. 3A, signals denoted by Va' indicate valid voltage signals that are electric AC voltage signals output at one cycle per second (1.0 Hz) by the magnetic-flux sensor each time the sensor senses the permanent magnet attached to the wobble plate, and signals denoted by Vb' indicate invalid voltage signals that are electric AC voltage signals output by the magnetic-flux sensor each time the sensor senses an approach of the above-mentioned steel support arm member. The valid and invalid voltage signals Va' and Vb' are sent to the electric binary circuit, by which these signals Va' and Vb' are subjected to a binary conversion with respect to a threshold level corresponding to an electric zero voltage level, and accordingly, these signals Va' and Vb' are converted into electric pulsive signals Vk as illustrated by FIG. 3B. The electric pulsive signals Vk are further subjected to a frequency division to obtain output pulsive signals Vk, illustrated by FIG. 3C, to thereby simplify the post-processing of the signals Vk'. The output pulsive signals Vk, contain erroneous signals Vn derived from the invalid voltage signals Vb', and accordingly, generate inaccurate information on the current capacity of the compressor.